1. Field of the Invention
This invention relates to gallium arsenide solar cells, and, more particularly, to methods of annealing out radiation damage in GaAs solar cells.
2. Description of the Prior Art
Radiation damage-induced degradation of the output of silicon solar arrays during the course of a space mission is a major problem in space power systems. Long term missions, and missions in or through regions of intense space radiation, will require either the ability to restore the array output to near beginning-of-life power or the use of solar arrays which exhibit little or no degradation.
The nature of radiation damage-induced defects, as well as their annealing kinetics, is only imperfectly known at present. Furthermore, there are currently no practical methods for cell annealing once a solar cell array has been deployed on a mission. An advanced technology concept with the potential to provide a radiation-hard solar power source incorporates gallium arsenide (GaAs) solar cells operating at elevated temperatures in concentrated sunlight. The concept, set forth in a Request for Proposal by NASA (RFP 3-183684Q, dated Apr. 2, 1980), utilizes three important features of GaAs solar cells compared to silicon: their apparent superior radiation hardness; a lower rate of cell output degradation as cell temperature increases; and lower annealing temperatures to remove radiation damage. Recent results indicate that it should be possible to operate GaAs cells at temperatures such that a significant amount of continuous thermal annealing of radiation damage occurs, while at the same time maintaining a reasonable level of cell output. A substantial loss of efficiency is, however, inherent to this system because of the relatively high temperature required when thermal annealing only is exploited.
Cover glasses are presently used to shield the solar cells from part of the damaging radiation. The effectiveness of such cover glass protection is, however, limited and leads to solar cell life limitations or weight penalties which are undesirable. Alternately, thermal annealing has also been proposed to limit solar cell radiation damage or recover from it. It has, however, not yet been implemented in any practical systems because of the relatively high annealing temperature and long annealing time required when thermal annealing alone is used.